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Description of problem:
Using a Dual Opteron on a Tyan S2880 motherboard and 6 2GB DIMMS only 8GB is seen by the kernel. On FC3 all the memory is seen and used but upgrading to FC4 only 8GB is seena and used. Something must have changed in 2.6.11.
Version-Release number of selected component (if applicable):
kernel-smp-2.6.11-1.1369_FC4
How reproducible:
Always
Steps to Reproduce:
1.Load FC4
2.Use the FC4 kernel on FC3
3.
Actual Results: Only 8GB is used
Expected Results: Should see all 12GB of memory
Additional info:

I tried the new 2.6.12 kernel from development
Linux version 2.6.12-1.1400_FC5smp (bhcompile@crowe.devel.redhat.com) (gcc
version 4.0.0 20050622 (Red Hat 4.0.0-13)) #1 SMP Thu Jun 23 23:32:58 EDT 2005
And I still have the same problem
Memory: 8106892k/12582912k available (2459k kernel code, 0k reserved, 1307k
data, 228k init)
I am not sure whether to look at the Tyan Motherboard or if I should download
the Kernel source and compile. Does RedHat put a limit in the Fedora Kernel at 8GB?

Your BIOS has a buggy SRAT table..
SRAT: Node 1 PXM 1 200000000-2ffffffff
This line tells us that there is 4GB at that address, but according to the e820
table..
BIOS-e820: 0000000100000000 - 0000000300000000 (usable)
there's really 8GB there.
Nothing we can do here, you'll have to complain to your motherboard vendor to
fix their BIOS. (Or possibly try changing options in the BIOS related to
SRAT/NUMA if there are any at all).

[This comment has been added as a mass update for all FC4 kernel bugs.
If you have migrated this bug from an FC3 bug today, ignore this comment.]
Please retest your problem with todays 2.6.12-1.1398_FC4 update.
If your problem involved being unable to boot, or some hardware not being
detected correctly, please make sure your /etc/modprobe.conf is correct *BEFORE*
installing any kernel updates.
If in doubt, you can recreate this file using..
mv /etc/sysconfig/hwconf /etc/sysconfig/hwconf.bak
mv /etc/modprobe.conf /etc/modprobe.conf.bak
kudzu
Thank you.

We saw something similar with a system here.
2.6.10-1.770_FC3smp uses uses the e820 info and 2.6.11-1.14_FC3smp uses the ACPI
SRAT memory info. Boot with acpi=off and all the memory will be detected (but
your system may be unstable so don't try to run that way).
I think Dave Jones was correct to say that you have a BIOS bug.